BERKELEY, CA — New experimental findings by Lawrence
Berkeley National Laboratory (Berkeley Lab) cell biologist Mary Helen
Barcellos-Hoff show that exposure to ionizing radiation creates a microenvironment
in the tissue surrounding breast cells that can cause even nonirradiated
cells and their progeny to become cancerous. The discovery suggests new
and possibly more effective means for preventing breast cancer.

Speaking in Boston at the annual meeting of the American Association
for the Advancement of Science (AAAS), Barcellos-Hoff described her study
in which a special line of nonirradiated, nonmalignant breast cells were
transplanted into irradiated mammary glands. Nearly 75 percent of the
transplanted glands developed tumors, and the effect persisted up to 14
days after the radiation exposure. Tumors developed in less than 20 percent
of the glands when Barcellos-Hoff transplanted the same type of cells
into nonirradiated mice.

Mary Helen Barcellos-Hoff, a cell biologist with
the Lawrence Berkeley National Laboratory, has shown that exposure
to ionizing radiation can cause breast cancer by pathways other than
genetic mutations.

"Our studies demonstrate that radiation elicits rapid and persistent
global alterations in the mammary gland microenvironment," says Barcellos-Hoff.
"We believe that these radiation-induced microenvironments lead to
changes in the physical characteristics (phenotypes) of cells and their
progeny that promote carcinogenesis. In other words, radiation exposure
can cause breast cancer by pathways other than genetic mutations."

Studies by Barcellos-Hoff and her research group indicate that one of
these alternative pathways is damage to the tissue that surrounds a breast
cell. This surrounding tissue, which includes a network of fibrous and
globular proteins called the extracellular matrix (ECM), normally acts
to suppress cells from becoming cancerous.

"Repairing damaged tissue so that it once again suppresses instead
of promotes carcinogenesis is a simpler strategy for stopping the cancer
process, compared to trying to repair individual damaged cells,"
says Barcellos-Hoff. "Our data is pointing to the tissue surrounding
breast cells as a primary target of ionizing radiation damage."

Ionizing radiation is a well-established carcinogen, but previous studies
of its cancer-causing effects have largely focused on damage to the breast
cells' DNA. If repaired improperly, this damage gives rise to genetic
mutations or chromosome damage that if passed on to daughter cells leads
to cancer. In that context, the question for medical researchers has been:
How do cells become cancerous?

Barcellos-Hoff has pursued a different tack. "It takes a tissue
to make a tumor," she says. "Cells don't become tumors without
cooperation from the surrounding tissue. Cancer is a process that occurs
at the tissue level and the question we ought to be asking is: How do
tissues become tumors?"

To answer that question, Barcellos-Hoff and her group, which includes
postdoctoral fellow Rhonda Henshall-Powell, have focused their attention
on the extracellular signaling that takes place between a cell and the
microenvironment of its surrounding tissue. Their studies and others have
shown that proper communications between the cell and its microenvironment
are crucial to normal functioning. The director of Berkeley Lab's Life
Sciences Division, Mina Bissell, has shown that breakdown in these communications
can initiate the cancer process or cause an abnormally high rate of apoptosis¾programmed
cell death¾another significant factor in the development of breast
and other cancers.

"Ionizing radiation is like a wound, in that it produces a defensive
response from the affected tissue. Usually this helps to protect undamaged
cells and eliminates those that have become abnormal," Barcellos-Hoff
says. "However, if there is too much damage, the defense response
can become a problem."

For example, exposure to just the right dose of ultraviolet radiation
will cause skin tissue to respond by producing melanin, the protective
skin-darkening pigment. Too much exposure at once, however, leads to sunburn,
and repeated exposures over time will damage the tissue, causing wrinkles
and possibly skin cancer. In the case of mammary glands exposed to low
doses of ionizing radiation, the surrounding tissue has been programmed
to send signals to the cells that would suppress genomic mutations and
cause cell apoptosis. But as the exposure intensifies, the defense program
becomes "corrupted" and the wrong signals get transmitted.

"We hypothesize that under certain conditions, radiation exposure
prevents normal cell interactions, which in turn predisposes susceptible
cells to genomic instability that can result in mutations," Barcellos-Hoff
says.

These two images of a special line of breast cells
compare
nonirradiated cells on the left to irradiated cells on the right.
Cell nuclei are dyed red, and E-cadherin, an important cell-to-cell
adhesion molecule, is dyed green. The nonirradiated cells adhere together
in a tightly organized clump, known as an acini, while the irradiated
cells, lacking adhesion, are disorganized. Berkeley researchers propose
that extracellular communication disrupted by the irradiation is the
cause.

In their study with cells transplanted into irradiated mammary glands,
Barcellos-Hoff and senior research associate Shraddha Ravani exposed specially
created epithelium-free glands (mouse epithelium develops postnatally
and is readily removed from the gland) to low-level radiation doses (4
grays, or 400 rads). Upon observing the persistent carcinogenic effects
on the transplanted cells, they established that the radiation damage
to the tissue was generating signals that altered how the cells' genomes
were expressed. This resulted in the creation of a new cell phenotype
with physical characteristics that were cued by the extracellular signals
to act cancerous. Breast cells acquiring the new phenotypes passed these
characteristics onto their daughter cells.

"Genomes are like the keys on a piano, in that the same keys can
be used to play a wide variety of music," says Barcellos-Hoff. "In
our studies, the ionizing radiation elicited changes in how the genomes
of the transplanted cells were being expressed by changing the extracellular
signals they were receiving."

Barcellos-Hoff and her colleagues now want to identify the altered signals
that are being sent from the irradiated tissue to the cells and determine
the mechanism by which these signals are destabilizing breast cell genomes.
To do so they are using a model of organized human breast cells developed
by Bissell.

In her AAAS talk Barcellos-Hoff also discussed the preliminary findings
of a study on which she is working in collaboration with Bissell and radiation
oncologist Catherine Park. The team has found that irradiated human breast
cells also show persistent phenotypic changes that affect their ability
to interact with other cells. Such behavior is typical of cancer cells.

Berkeley Lab is a U.S. Department of Energy national laboratory located
in Berkeley, California. It conducts unclassified scientific research
and is managed by the University of California.